package oj;

public class Num022 {
    public ListNode detectCycle(ListNode head) {
        if(head == null){
            return null;
        }
        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                break;
            }
        }
        if(fast == null || fast.next == null){
            return null;
        }
        fast = head;
        while (fast != slow){
            fast = fast.next;
            slow = slow.next;
        }
        return fast;
    }
}
